Known common rail fuel systems include an accumulator volume or common rail which is charged with fuel at high pressure by means of a high pressure fuel pump. Fuel at high pressure is supplied by the common rail to a plurality of injectors, each of which is arranged to inject fuel into an associated engine cylinder.
Each injector includes an injection nozzle having a valve needle which is spring biased towards a closed position in which it is seated against a valve needle seating. Injection is controlled by controlling fuel pressure (control pressure) in a control chamber by means of a two way nozzle control valve. The control chamber is at the back end of the valve needle and communicates continuously with a high pressure supply passage from the common rail.
The nozzle control valve is operable between an open position, in which the control chamber communicates with a low pressure drain, and a closed position in which communication between the control chamber and the drain is broken. In order to commence injection, the nozzle control valve is moved into its open position so as to open communication between the control chamber and the low pressure drain, thereby causing fuel pressure within the control chamber to be reduced and thus allowing the valve needle to lift from its seating. In order to terminate injection the nozzle control valve is moved into its closed position, closing communication between the control chamber and the low pressure drain. Due to the continuous flow of fuel into the control chamber, high fuel pressure is re-established within the control chamber, which causes the valve needle to be re-seated to terminate injection.
When the valve needle reaches its maximum lift position during injection, communication between the control chamber and the low pressure drain is inhibited substantially instantaneously. The abrupt termination of the spill flow from the control chamber into the low pressure drain can cause a collapsing cavity and/or pressure spike. The resulting local low pressure regions and reflected pressure waves may cause cavitation erosion, for example proximal the valve seat of the nozzle control valve. The operation of the nozzle control valve can also be affected.
To highlight the impact of the valve needle reaching its maximum lift position a fuel delivery gain curve C2 is shown in FIG. 1A and a fuel delivery stability curve C1 is shown in FIG. 1B. The fuel delivery gain curve C2 represents the fuel injected from the nozzle during injection. A non-linearity ‘A’ is evident on the gain curve approximately 800 μs from the actuation of the injector. The fuel delivery stability curve C1 shows a spike ‘B’ representing an increase in the shot to shot fuel delivery quantity instability that is coincident with the non-linearity ‘A’ on the fuel delivery gain curve C2. This coincident spike ‘B’ represents a non-linearity and instability which is the result of the abrupt termination of the spill flow from the control chamber into the low pressure drain when the nozzle valve needle reaches maximum lift ‘C’ causing a cavity to form behind the spill flow pulled by the fluid momentum of the flow within the spill drain. This low pressure cavity then collapses as the fluid flow reverses within the spill drain. At the point of cavity collapse a local high pressure is generated within the spill drain below the nozzle control valve (NCV) bottom seat. It is this local high pressure at the point of cavity collapse acting on the underside of the NCV valve pin affecting the motion of the nozzle control valve closure back onto the bottom seat causing the non-linearity.
It has been proposed to incorporate a restriction in the secondary drain line of prior art systems. A precisely sized orifice can restrict flow in and out of the area below the NCV bottom seat. However, this arrangement requires high precision geometry or flow conditioning within the spill port itself.
It is one object of the present invention to provide an improved fuel injector, suitable for use in common rail or unit injector type fuel systems, which overcomes or alleviates at least one of the aforementioned problems.